Afterburner seals with heat rejection coats

ABSTRACT

A method for applying a coating system that is applied to a surface of a component for preventing or at least substantially preventing interdiffision between the component surface and a protective thermal layer applied to the component surface when the thermal layer is exposed to elevated temperatures. The method includes applying a carrier layer containing aluminum to the component surface. Next, the layer is heated to a first predetermined temperature for a first predetermined period of time in the substantial absence of oxygen to bond the aluminum with the component surface, the heat dissolving the carrier portion of the aluminum layer. The remaining portion of the aluminum layer is then heated to a second predetermined temperature for a second predetermined period of time to form an oxidized aluminum layer. Finally, at least one protective thermal layer is applied over the oxidized aluminum layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to application Ser. No. 10/324,704, filedcontemporaneously with this Application on Dec. 20, 2002, entitled“COMBUSTION LINER WITH HEAT REJECTION COATS” assigned to the assignee ofthe present invention and which is incorporated herein by reference, andto application Ser. No. 10/325,000, filed contemporaneously with thisApplication on Dec. 20, 2002, entitled “TURBINE NOZZLE WITH HEATREJECTION COATS” assigned to the assignee of the present invention andwhich is incorporated herein by reference.

FIELD OF THE INVENTION

Thus there is a need in the art for an inexpensive, lightweight means toprevent interdiffusion between component surfaces and heat rejectioncoatings. The present invention relates generally to heat rejectioncoats applied to component surfaces exposed to high temperatures andmore particularly to providing a diffusion barrier sub-coating prior toapplying the heat rejection coats to the component surfaces to stabilizeand preserve the heat rejection coats.

BACKGROUND OF THE INVENTION

Components exposed to elevated temperatures and mechanical stresses,such as aircraft engines which typically employ nickel, iron or cobaltbased superalloys, require protective coatings from corrosion and fromthe high operating temperatures to achieve reliable operation forextended periods of time. More specifically, component surfaces havingmetallic heat rejection coatings, such as platinum, gold or rhodiumwhich may be sandwiched between a pair of stabilizing layers, such astantalum, that are exposed to radiative flames exhibit both measurabletemperature decrease and increased service life compared to uncoatedcomponent surfaces. These heat rejection coatings achieve thistemperature decrease by effectively reflecting the radiative energy awayfrom the component surface. Accordingly, it is highly desirable to applythese heat rejection coatings to similarly exposed surfaces. However,this is not possible for certain metal alloy parts, such as afterburnerseals, which may be regularly exposed to temperatures exceeding about788° C. (1450° F.). At this temperature range, the heat rejectioncoating interdiffuses with the underlying component surface, orsubstrate. In essence, a portion of the heat rejection coating materialmigrates into the component substrate material. This interdiffusioncauses the reflective heat rejection surface to become a radiationabsorber, losing or at least substantially losing its ability to reflectradiative energy, resulting in a reduction of its ability to decreasecomponent surface temperature, thereby decreasing the service life ofthe component. Therefore, a means to prevent the interdiffusion betweencomponent surfaces and the heat rejection coatings is highly desired.

One method to prevent this interdiffuision is the provision of a barriercoating applied between the component surface and the heat rejectionsurface. A variety of these barrier coatings are known in the art andinclude paint-on dielectric oxides, chemical vapor deposited oxides andbaked-on rare earth oxides. However, none of these barrier coatingconstructions may be utilized in this application because they eitherare inefficient in preventing diffusion or lose their adhesiveproperties at higher temperatures.

Alternately, it has been shown that nozzles may be covered with a thickmacroscopic coating of a ceramic thermal barrier coating, referred asTBC, which is also known as “smooth coat,” commonly employing a TBCcomposition referred as “(AJ11).” U.S. Pat. Nos. 5,624,721 and 5,824,423are directed to methods which employ aluminum bond coat layers forsecuring TBC coatings. While heat rejection coatings have been shown toremain intact when applied over these TBC coatings, the thick TBCcoatings are expensive to manufacture and apply and are extremely heavy,effectively limiting their application to aerospace components.

Thus there is a need in the art for an inexpensive, lightweight means toprevent interdiffusion between component surfaces and heat rejectioncoatings.

SUMMARY OF THE INVENTION

The present invention is directed to a method for applying a coatingsystem that is applied to a surface of a component for preventing or atleast substantially preventing interdiffusion between the componentsurface and a protective thermal layer applied to the component surfacewhen the surface is exposed to elevated temperatures. The methodincludes first applying an aluminum-based material to the componentsurface. In addition to aluminum, this material may include a carriermaterial and a binder, both of which are typically organic when thematerial is a paint. Next, the layer is heated to a first predeterminedtemperature for a first predetermined period of time in the substantialabsence of oxygen to metallurgically bond the aluminum with thecomponent surface, the heat volatizing the carrier and binder portion ofthe aluminum layer. The remaining portion of the aluminum layer is thenheated to a second predetermined temperature for a second predeterminedperiod of time in the presence of oxygen to form an oxidized aluminumlayer alumina. Finally, at least one protective thermal layer is appliedover the alumina.

The aluminum layer can be applied by standard commercially availablealuminide processes whereby aluminum is reacted at the substrate surfaceof the component to form an aluminum or aluminum-containing compositionwhich provides a reservoir for the growth of the aluminum oxidationlayer. This aluminum layer is typically and predominantly aluminum, butmay also be combined with other metals, including nickel, cobalt andiron as well as aluminum phases of nickel, cobalt and iron, or may beformed by contacting an aluminum vapor species or aluminum rich alloypowder with the component substrate and depositing the aluminum on thesubstrate surface. This layer is typically metallurgically bonded to thesubstrate and may be accomplished by numerous techniques, including apack cementation process, over-the pack processing, spraying, chemicalvapor deposition, electrophoresis, sputtering, vapor phase aluminidingand slurry sintering with an aluminum rich vapor and appropriatediffusion heat treatments. Aluminum will form highly stable refractoryoxide layers at the operating temperature of hot section componentswhich are tightly adherent and cohesive and thus effective to blockincursions of corrosive chemical agents into the component substrate, solong as the aluminum oxide layer remains intact while preventing themigration of substrate elements outward. When a coating is applied overthe alumina, it prevents the coating elements from migrating inward orthe substrate elements from migrating outward. In other words, the oxidelayer will act as a barrier to prevent interdiffusion of elements acrossit.

A primary advantage of the present invention is an inexpensive,lightweight means to prevent interdiffusion between the substrate andheat rejection coatings applied over the substrate surfaces exposed toelevated temperatures.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a jet engine depictingcomponent regions having surfaces suitable for employment of the methodfor applying a coating system of the present invention.

FIG. 2 is an enlarged partial cross sectional view taken from FIG. 1 ofthe afterburner region after a paint coating containing aluminum hasbeen applied.

FIG. 3 is the enlarged partial cross sectional view of FIG. 2 ofaluminum diffusing into the substrate by application of heat in thesubstantial absence of oxygen.

FIG. 4 is the enlarged partial cross sectional view of FIG. 3 offormation of an aluminum oxide layer forming from migration of thediffused aluminum from within the substrate by the application of heatin the presence of oxygen.

FIG. 5 is the enlarged partial cross sectional view of FIG. 4 afterapplication of a heat rejection coating over the aluminum oxide layer.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a jet engine 10 is provided having a hotsection 14 which advantageously utilizes the coating system and methodof application of the present invention. Engine 10 includes in serialflow relation proceeding in a direction from an inlet 16 to exhaustnozzle 30, through a low pressure compressor 18, a high pressurecompressor 20, a combustor 22, a high pressure turbine 24, a lowpressure turbine 26, an afterburner 28, and terminating in exhaustnozzle 30. Afterburners 28 are optional items used in militaryapplications. Air entering inlet 16 is compressed by compressors 18, 20before reaching combustor 22 where the highly compressed air is mixedwith fuel and ignited. This air/fuel exhaust mixture is then propelledthrough turbines 24, 26 which are urged into rotation by the passingmixture of hot gases to likewise rotatably drive respective compressors18, 20, since these components are connected to a common drive shaft.Upon reaching optional afterburner 28, fuel is introduced into themixture stream to augment thrust, utilizing the unburned oxygen in theexhaust gas to support combustion. Afterburners are typically used inmilitary aircraft. They increase the speed of the aircraft for shortperiods of time, by injecting fuel into the hot exhaust gas stream whereit is combusted, thereby providing additional thrust. The temperature ofthe afterburner flame can exceed about 1,700° C. (about 3,100° F.), sothe burners of afterburner 28 are directed radially inward so that atleast a portion of the mixture from turbines 24, 26 flow past the wallof this region, helping maintain wall temperatures at somewhat reducedtemperature levels. The resultant increased temperature of the exhaustgas increases its velocity as it leaves exhaust nozzle 30, providingincreased engine thrust.

Afterburner 28 components, such as the seals, are nonetheless subjectedto significant radiative heat from afterburner flames despite the burnerorientations and will greatly benefit from the present invention.

Referring to FIG. 2, a sub coating layer 52 which is a carrier layercontaining aluminum applied over the substrate 50 shall now bediscussed. In the preferred embodiment, sub coating layer 52 is formedfrom application of commercially available spray paint, such as Krylon®No. 1404, manufactured by Sherwin-Williams Company, although comparablepaints from other manufacturers could likely also be used. It isrealized that any of the conventional methods to form this aluminumlayer previously discussed could also be employed. Carrier materialcontained within the layer permits sub coating layer 52 to be sprayedover a substrate surface 51 of a substrate 50, such as an afterburnerseal. Sub coating layer 52 may be applied to substrate surface 51 ofsubstrate 50 in a manner substantially similar to that employed to applya coat of paint to an article sufficient to “cover” the article. Inother words, by applying one, preferably two, coats of paint from acommercially available spray can to surface 51, sub coating layer 52contains an amount of aluminum particles 54 sufficient to ultimatelyform an aluminum oxide layer 56 (FIGS. 4, 5) on surface 51 as will bediscussed in greater detail below.

Further referring to FIG. 2, which is a partial cross-sectional view ofa coated afterburner 28 seal, aluminum particles 54 carried within subcoating layer 52 are suspended within binder materials (not shown) inthe paint formulation which bind the layer to the seal surface andprevent aluminum particles 54 from combining with oxygen to formaluminum oxide. These binder materials will be removed, i.e., vaporized,by a first thermal treatment step that will be further discussed belowand is not otherwise addressed herein. Aluminum particles 54 preferablyhave a platelike morphology that will be substantially oriented parallelto surface 51. More preferably, aluminum particles 54 are about ½ micronin thickness and are substantially equally distributed within subcoating layer 52. These particles preferably have an aspect ratio ofbetween about 100:1 to about 10:1, 20:1 being the most preferred.Particle aspect ratios exceeding this upper range are difficult to applyby spraying, and ratios below this lower range have decreased“coverability.”

In preparation for this first heating step, substrate 50 is placed in anenvironment, such as a substantially fluid-tight oven or heatingchamber, having an extremely low oxygen partial pressure, or having asubstantial absence of oxygen. This may also be accomplished by anenvironment filled with inert gases, such as argon, helium, or evenhydrogen or nitrogen, although a vacuum of sufficient magnitude may beemployed. Once substrate 50 has been placed in the desired environmentor the conditions of the desired environment have been achieved,substrate 50 and sub coating layer 52 are subjected to a first heattreatment. During the first heat treatment, the environment temperaturereached and maintained for the duration of the first heat treatment mayrange from about 600° C. (1,100° F.) to about 1,000° C. (1,830° F.). Onehaving skill in the art realizes that the duration of the first heattreatment varies depending upon the temperature selected, since the rateof diffusion of aluminum is exponentially affected by temperature, forexample, substrate 50 will typically require about fifty hours ofexposure at about 600° C. (1,100° F.), or about one hour of exposure atabout 1,000° C. (1,830° F.) to achieve substantially the same results,i.e., same depth of diffusion. Therefore, any number of heat/exposurecombinations may be employed as a matter of manufacturing convenience,so long as the results achieved substantially mirror the results of the600° C./1,000° C. (1,110-1,830° F.) exposures just described. Once thisfirst heat treatment has been completed, referring to FIG. 3, asignificant amount of diffused aluminum 55 is diffused into substrate50, forming an alloy with the interdiffused substrate.

Referring to FIG. 4, after the first heat treatment has been completed,substrate 50 is subjected to a second heat treatment. In essence,termperature/exposure of the second heat treatment is substantiallysimilar to that previously described for the first heat treatment.However, the major difference between the two heat treatments is thatthe second heat treatment is performed in the presence of oxygen. Thisoxygen exposure promotes the formation of an aluminum oxide layer 56along the surface of substrate 50. Aluminum 54 remaining on the surfaceoxidizes and a portion of the diffused aluminum 55 that had previouslydiffused into substrate 50 during the first heat treatment migrate tothe substrate surface so as to form a continuous tightly adherentaluminum oxide layer. Preferably, aluminum oxide layer 56 is from aboutone to about ten microns thick, although this layer may permissibly beup to about ten mils (0.010 inches) in thickness.

Referring to FIG. 5, after aluminum oxide layer 56 has been formed, asmooth protective thermal coating may be applied. This coating may bechemical vapor deposited via a reagent of tantalum ethoxide, which flowsinto the environmental chamber containing substrate 50. It is criticalthat the protective thermal layer be smooth to controllably reflectradiative energy away from substrate 50 and into the gas stream.Otherwise, the radiative energy may be scattered or reflected towardunintended regions of the engine, with adverse results. Upon contactingsubstrate 50, the tantalum ethoxide deposits a tantalum oxide layer 58and ethanol by-products. Similarly, a platinum oxide layer 60 is formedby chemical vapor deposition which is then followed by application of asecond tantalum oxide layer 62 that is applied over platinum oxide layer60. The sandwiching tantalum oxide layers 58, 62 add stability toplatinum oxide layer 60, especially at higher temperatures. The ethanolby-products, being volatile, are readily removed. Other noble metallayers that may be applied, in addition to platinum and tantalum,include palladium and rhodium. Other protective layers that may beapplied in addition to tantalum oxide are titanium oxide, silicon oxide,zirconium oxide, haffiium oxide, aluminum oxide, chromium oxide andmixtures thereof.

Successful exposure testing of coupons, typically lengths of materialapproximately one inch in diameter, have been conducted. Such testingtypically consists of exposing the coupon to a heat-up period fromambient to a first desired temperature level, requiring a time interval,such as about twenty minutes, holding the coupon at the first desiredtemperature level for another time interval, such as about fortyminutes, before cooling-down the part in a manner similar to theheating-up period, and repeating these heat-up cool-down cycles to/fromthe first temperature for a predetermined number of times, typicallyseveral hundred. If the coupon survives the first temperature, thetemperature is raised by some increment, typically several hundreddegrees, and similarly cycled until the coupon coating spalls. Thepresent invention has exceeded about 1,000° C. (1800° F.) which istypically the upper temperature range of temperature an afterburner sealwill see in service.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A superalloy afterburner seal having high temperature capabilitycomprising: an additive layer of alumina applied to an exposed surfaceof the superalloy seal; and a noble metal layer applied over the aluminaproviding a highly reflective layer, the seal further characterized by asubstantial absence of a subsequent ceramic layer.
 2. The afterburnerseal of claim 1, wherein the noble metal layer is selected from a groupconsisting of platinum, palladium and rhodium.